The present invention relates to injection molding and in particular to a melt transfer system for a stack mold.
In a stack mold, pressurized melt must be conveyed across a parting line between two platens that are displaceable relative to each other. A melt transfer system is required in order to control the flow of the pressurized melt depending on whether the platens are in contact with each other or are separated.
A valve gated melt transfer system, such as is disclosed in U.S. Pat. No. 4,244,909, controls the flow of melt between platens using a combination of valve gate units. A disadvantage with valve gated melt transfer systems is that pressure variances within the melt passages are experienced due to the presence of valve pins.
A thermal gated melt transfer system, such as disclosed in U.S. Pat. No. 4,586,887, controls the flow of melt between platens using a combination of heated nozzles. A disadvantage with thermal gated melt transfer systems is that the flow of pressurized melt is impeded by the relatively small diameter gate defined in each heated nozzle. Furthermore, there is a delay associated with cooling and remelting the melt in the gate for each molding cycle.
There is a need for a melt transfer system to be developed for stack molds that overcomes the above problems and that is relatively simple in its construction and effective in its operation.
The improved stack mold system is particularly needed for large scale molding operations involving large volumes of melt being transferred at one time. In such cases, it is desirable to have larger diameter melt passages that are not impeded by central valve pins or other obstructions that interfere with the flow of melt within the passages. It is also desirable in such large scale molding operations that drool be controlled in a manner that does not solely rely upon decompression at the injection nozzle as it has been found difficult to achieve a sufficient decompression over long lengths of passages.
The present invention provides an improved system and method for conducting pressurized melt between platens in a stack mold.
In one aspect the invention provides a melt transfer bushing comprising:
a body defining a channel for conducting a pressurized melt between a shear gate at one end of said channel and an aperture at another end of said channel, wherein said shear gate is defined in a plane that is not parallel to the plane in which said aperture is defined.
In another aspect the invention provides a melt transfer bushing comprising:
a body defining a channel for conducting a pressurized melt between a shear gate at one end of said channel and an aperture at another end of said channel, wherein said shear gate is defined by a circumferential edge forming an acute angle relative to the plane of said shear gate. A melt transfer bushing comprising:
a body defining a channel for conducting a pressurized melt between a shear gate at one end of said channel and an aperture at another end of said channel, wherein said shear gate is defined by a circumferential edge forming an acute angle relative to the plane of said shear gate.
In another aspect the invention provides a melt transfer bushing comprising:
a core formed of an insulative material, said core defining a channel for conducting a pressurized melt between a shear gate at one end of said channel and an aperture at another end of said channel; and
a shell formed of a conductive material surrounding said core between said shear gate and said aperture.
In another aspect the invention provides a melt transfer system for a stack mold having a first platen and a second platen, the second platen moving between an open position and a closed position relative to the first platen and cooperating with the first platen to define at least one mold cavity when in said closed position, the melt transfer system comprising:
a first transfer bushing disposed in the first platen defining at least a portion of a first runner passage for conducting pressurized melt from a melt inlet to a first shear gate, said first shear gate being defined in a first shearing surface disposed in said first transfer bushing in a plane extending generally parallel to the direction of opening and closing movement of said first and second platens; and
a second transfer bushing disposed in the second platen defining at least a portion of a second runner passage for conducting pressurized melt from a second shear gate to a distribution manifold, said second shear gate being defined in a second shearing surface disposed in said second transfer bushing in a plane extending generally parallel to said first shearing surface;
wherein said first and second shear gates are in communication to facilitate flow of pressurized melt from said first runner passage to said second runner passage when the first and second platens are in a closed position, and wherein said pressurized melt is sheared at said first and second shear gates by said first and second shearing surfaces when the first and second platens move from a closed position to an open position.
In another aspect the invention provides a melt transfer system for a stack mold having a first platen and a second platen, the second platen moving between an open position and a closed position relative to the first platen and cooperating with the first platen to define at least one mold cavity when in said closed position, the melt transfer system comprising:
a first runner passage defined in the first platen for conducting a pressurized melt from a melt inlet to a first shear gate;
a second runner passage defined in the second platen for conducting said pressurized melt from a second shear gate to a distribution manifold;
a first bushing assembly defining at least a portion of said first runner passage and having a first bearing surface into which said first shear gate is defined;
a second bushing assembly defining at least a portion of said second runner passage and having a second bearing surface into which said second shear gate is defined, said first and second bearing surfaces contacting each other when said first and second platens are in a closed position;
means for moving at least one of said first and second bushing assemblies between a flow position, where said first and second platens are in a closed position and said first and second shear gates are in communication to facilitate flow of pressurized melt from said first runner passage to said second runner passage, and a shear position, where melt at said first shear gate is sheared by said second bearing surface an melt at said second shear gate is sheared by said first bearing surface to prevent flow of pressurized melt from said first runner passage to said second runner passage.